If memory serves me correctly: If both the devices (speaker selector and volume control) are 'impedance matching' (Niles calls it impedance magnifying, btw) you could very well have audibly impaired volume.

Impedance matching devices in general do decrease the amount of power that each speaker receives (and therefore the speaker's maximum loudness). If the mains are more efficient than the in ceiling speakers (a likely scenario) then the perceived loudness at the same power level would be lower in the kitchen.

Some more details would certainly help with a diagnosis:

1) What gauge is the speaker wire on the 50 foot run to the kitchen?

2) What amplifier are you using? Is it capable of handling a 4 Ohm load?

3) Are you using the protection circuit on the speaker selector AND the impedance magnification setting on the volume control? This is a problem I had before and a likely culprit.

4)If so, Remove the volume control from the wall, and set the impedance magnification to 1x

5) If this has no effect, take the volume control out of the loop completely and test again.

6) If volume is better, you might want to get a NON impedance matching volume control since the speaker selector is already doing that for you.

Hope this helps; let us know how it goes...
Tony.